1. Field of the invention
The present invention relates to the preparation of adipic acid by hydrocarboxylation to pentenic acids and, more especially, to the preparation of adipic acid by reacting water and carbon monoxide with at least one pentenic acid, in the presence of a rhodium-based catalyst and of at least one iodine-containing promoter therefor.
2. Description of the Prior Art
Published European Patent Application No. 188,209 describes a process for the preparation of linear dicarboxylic acids, in particular adipic acid, by reacting unsaturated monocarboxylic acids, in particular 3-pentenoic acid, with carbon monoxide and water in the presence of a rhodium-based catalyst and an iodine-containing promoter, the reaction being conducted in a solvent such as methylene chloride at a temperature of 100.degree. to 240.degree. C. and under a total pressure of from 14 to 240 atm; a temperature of from 150.degree. to 180.degree. C. and a total pressure of from 24 to 40 atmospheres are considered to be preferable. The partial pressure of carbon monoxide typically ranges from 10 to 35 atm and preferably from 10 to 17 atm. The selection of the solvent is considered to be critical according to such '209 application and it too is considered that solvents such as acetic acid are undesirable because of the low degrees of linearity of the final products obtained in their presence.
Similarly, it too is noted that nonpolar solvents such as cyclohexane and toluene are themselves also undesirable because of their propensity to promote directly the formation of branched final products and, indirectly, saturated monocarboxylic acids.
Published European Patent Application No. 0,274,076 describes a process for the preparation of linear carboxylic acids by hydroxycarboxylation of unsaturated esters or of terminally unsaturated alkenes having from 4 to 16 carbon atoms in the presence of a rhodium-based catalyst and an iodine-containing promoter therefor. The reaction is conducted in a solvent selected from among methylene chloride, 1,2-dichloroethane and aromatic solvents, such selection being of no consequence, and an aliphatic or aromatic acid having a pKa ranging from 4.2 to 5.2 is present as a reaction accelerator. The partial pressure of the carbon monoxide ranges from 10 to 200 and preferably from 13 to 20 atm.
However, when employing pentenic ester starting materials, the formation of methyl monoadipate is characteristic thereof.